This field guide was written by Dr. Nicole Pietrasiak, now with John Carroll University. Biological soil crusts (also known as microbiotic, cryptobiotic, cryptogamic, microphytic soil crusts, or, for short, biocrusts) are crucial components of terrestrial ecosystems. It provides a dichotomous key with details, explanations, and illustrations for biocrust community types. It is best aligned with conditions in areas such as the California Coastal Plain, Southern California Mountains, Sierra Nevada Mountains, Southern Nevada Basin and Range, Mojave Desert, and the Lower Colorado Desert.